Dynamic range is the ratio of intensity of the highest luminance parts of an image scene and the lowest luminance parts of a scene. For example, the image projected by a video projection system may have a maximum dynamic range of 300:1.
The human visual system is capable of recognizing features in scenes which have very high dynamic ranges. For example, a person can look into the shadows of an unlit garage on a brightly sunlit day and see details of objects in the shadows even though the luminance in adjacent sunlit areas may be thousands of times greater than the luminance in the shadow parts of the scene. To create a realistic rendering of such a scene can require a display having a dynamic range in excess of 1000:1. The term “high dynamic range” (HDR) means dynamic ranges of at least 800:1.
Conventional display technology, using direct-lit local dimming (as an example, described by U.S. Pat. No. 8,277,056, “Locally Dimmed Display,” incorporated herein for all purposes), is capable of rendering images in a manner that faithfully reproduces high dynamic ranges. This is accomplished by independent modulation of light sources, as well as modulation by one or more liquid crystal panels, for improved contrast. However, a direct-lit panel 100 cannot do so in a form factor sufficiently thin for many applications (e.g., a cellular telephone display). As shown in FIG. 1A, the width 102 of a direct-lit panel stacks the light sources, liquid crystal panel, and intervening optics (such as, a diffusion layer).
As an alternative to direct-lit panels, edge-lit technology (as an example, described in U.S. Pat. No. 8,446,351, “Edge Lit LED based Locally Dimmed Display,” incorporated herein for all purposes) is employed with mixed results. As shown in FIG. 1B, an edge-lit panel 150 is thinner by not stacking a light source, and width 152 driven by the liquid crystal panel, light guide, and any intervening optics (not shown). That said, edge-lit panel 150 suffers from noticeably reduced HDR performance because light source modulation is row dependent and light intensity decreases along the length of the light guide (e.g., non-uniformity as a function of distance from the light source).
Accordingly, a need exists for a compact (e.g., thin) local dimming display capable of reproducing a wide range of light intensities.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.